Mechanical gyros, ring laser gyros, and fiber optic gyros are widely employed to provide a stable reference or a rate sensing device for navigation purposes. The fiber ring interferometer disclosed in the referenced application employs single mode optical couplers to reduce the problems associated with the ambient effects of temperature change, vibrations, shock, etc., on the heretofore employed beam-splitters and lens arrangement.
Shown as prior art in FIG. 1 of the drawings, a fiber interferometer 10 of the referenced application has a several hundred meter length single mode optical fiber 11 arranged in a ring-shaped configuration which encircles an area 12 of predetermined dimensions. A pair of beamsplitters/couplers 13 and 14 couple coherent light from a laser 15, and a pair of photodetectors 16 and 17 monitor the interference fringes attributed to changes of the rate of rotation of the interferometer.
The output of the laser is a continuous signal split into two parts by the pair of beamsplitters so that the laser energy circulates in opposite directions through the long single mode fiber. The beams are recombined by the beamsplitter/couplers and the output optical power is measured using the photodetectors. The magnitudes of the optical signals are measured at the detectors and are 180.degree. out of phase. The measured signals are the normalized difference of the output currents of the photodetectors which provide the output signals of the interferometer.
However, effects of variation of the response of the laser, and the photodetectors, can give erroneous indications of fringe shift and hence, angular rotation. For example, the output signal could drift for a constant applied rotation rate due to photodetector and laser response changes in response to ambient temperature changes or ageing.
An existing method of determining the fringe shift due to rotation rate variation relies upon a d.c. technique which does not refer the measured signal to its zero reference level. This approach suffers from the degrading effects of 1/f noise (this noise factor increases to objectionable levels as f approaches a d.c. value).
Thus, there is a continuing need in the state-of-the-art for a technique for calibrating or otherwise determining the zero rotation rate reference levels for fiber interferometers which is capable of determining changing response in the active elements, the photodetectors and the laser.